1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) system. More particularly, the present invention relates to a method and apparatus for performing compensation of channel characteristics and demapping to a soft metric value using an estimated channel impulse response during coherent demodulation.
2. Description of the Related Art
In an OFDM system, a whole frequency band is divided into a plurality of subcarrier bands. Subchannel signals are mapped to the subcarriers by Inverse Fast Fourier Transform (IFFT) for transmission. During a reception operation, a received OFDM signal is divided into the subchannel signals by Fast Fourier Transform (FFT). Similar to other systems using radio channels, the OFDM system estimates a channel impulse response H representing the channel characteristics from a transmitter to a receiver and performs channel compensation for a received signal Y based on the channel impulse response.
Two modes of equalization, such as Zero-Forcing (ZF) and Minimum Mean Square Error (MMSE), are available for channel compensation in OFDM. Despite the advantage of simplicity, the ZF equalization does not compensate for noise. This lack of compensation leads to performance degradation. Alternatively, the MMSE equalization does provide noise compensation. However, it has drawbacks because there is a requirement for an estimation of standard noise deviation. The two algorithms commonly compensate for channel characteristics that the received signal assumes, before demodulation through symbol demapping.
FIG. 1 is a block diagram of a receiver in a conventional OFDM system.
Referring to FIG. 1, an Analog-to-Digital Converter (ADC) 104 converts an analog signal received through an antenna 102 to a digital signal. An FFT processor 106 transforms the digital signal to a frequency-domain signal. Since the frequency signal contains the effects of a channel, an equalizer 108 is included after the FFT 106 in order to eliminate the channel effects before decoding by a decoder 112.
The equalizer 108 eliminates the channel effects from the received signal by division operations. A symbol demodulator 110 equivalent to a symbol demapper demaps the channel-compensated signal received from the equalizer 108 to a signal point on a signal constellation corresponding to a predetermined modulation scheme, and to in-phase (I channel) and quadrature-phase (Q channel) soft metric values representing the signal point. The decoder 112 recovers the original information bits by decoding the soft metric values.
As stated before, the equalizer 108 performs division operations for channel compensation. The following description is made of the operation of the equalizer 108 in the context of ZF that facilitates simple hardware implementation. ZF is expressed asyk/Ĥk=Ĥk*yk/(Ĥk*Ĥk)  (1)where yk denotes a received signal and Ĥk denotes a channel impulse response representing estimated channel characteristics.
FIG. 2 is a detailed diagram illustrating the structure of a conventional equalizer.
Referring to FIG. 2, the equalizer includes complex multipliers 202 and 204, a complex multiplier 206, and dividers 208 and 210. The complex multipliers 202 and 204 multiply the I-channel and Q-channel components of a received signal yk corresponding to a kth modulation symbol, FFT_outk—I and FFT_outk—Q by the I-channel and Q-channel components of the channel impulse response Hk of the kth modulation symbol, Ch_Est_outk—I and Ch_Est_outk—Q. The complex multiplier 206 calculates the power of the channel impulse response Hk. The dividers 108 and 210 divide the products received from the complex multipliers 202 and 204 by the power of the channel impulse response Hk and output the resulting channel-compensated I-channel and Q-channel components Est_Sym_outk—I and Est_Sym_outk—Q.
Referring to FIG. 1, the symbol demodulator 110 determines soft metric values corresponding to Est_Sym_outk—I and Est_Sym_outk—Q based on the signal constellation and a reference point (a) which defines a minimum distance between signal points of the signal constellation. FIG. 3 illustrates a 16-ary Quadrature Amplitude Modulation (16QAM) signal constellation for symbol mapping and demapping. The horizontal axis represents the I channel and the vertical axis represents the Q channel. As illustrated in FIG. 3, 16 4-bit signal points corresponding to 16 modulation symbols have a minimum distance of 2a according to one another. Preferably, (a) is referred to as the reference point of the signal constellation.
The symbol demodulator 110 determines the soft metric values using Est_Sym_outk—I and Est_Sym_outk—Q and the reference point (a). Conventionally, the reference point (a) is a constant, for example, ‘1’. While 16QAM has been described, it is shown for illustrative purposes and it is to be understood that the same signal constellation applies to any modulation scheme with a higher order equal to or higher than that of Quadrature Phase Shift Keying (QPSK).
The above-described conventional technology suffers from two problems. One is the requirement for divisions. Implementation of complex divisions in hardware is as complex as multiplication by five times. The other problem is that the division of a received signal by channel power gives a reliability level to the signal which is not suitable for decoder input. When a received signal (a low-reliability signal) affected by a channel with low signal power and high noise power is divided by the power of the channel, the decoder incorrectly decides that the received signal is highly reliable. The resulting decoding errors cause an error flow even in a high Signal-to-Noise Ratio (SNR) environment.
Accordingly, there is a need for an improved technique and apparatus for performing channel compensation and symbol demodulation to overcome the requirement for divisions and a low-reliability signal.